Aerosol characterizing instruments generally require highly focused particle beams with little or no transmission losses. In addition, they need to interface to the sampling environment with a very high sampling rate so that more aerosol particles can be collected and sensitivity can be improved. Aerodynamic focusing lens stacks have been shown to generate highly focused aerosol particle beam into vacuum, and have been used effectively for various aerosol studies [1]. Current focusing lens stacks, however, operate on small particle diameters [4] and at low pressure and low flow rate. By design, aerodynamic focusing lens stacks for aerosol particles in the range of 0.5 um to 10 um can only operate at low flow rate and low pressure due to the low Reynolds numbers required for each focusing lens in order to maintain laminar flow within the lens stack. And the orifice sizes have to be kept below one centimeter and above 100 um in order to be machined with acceptable tolerances and aligned in an inlet system. As such, the low pressure and low flow rate make it fairly difficult to interface aerodynamic focusing lens stacks with an aerosol source at atmosphere pressure. Traditionally, single critical orifice devices have been used to interface lens stacks to the atmospheric pressure environment, where the dimensions of the orifices are defined by the pressure required by the lens stack. Due to the coupling between pressure and flow rate however, critical orifices yield a very poor sampling efficiency when the sampling flow is less than 0.05 L/min, resulting in a very small number of particles transmitted through the entire system.
What is needed therefore is an aerosol focusing system (AFS) having a large-particle focusing inlet with a high sampling rate that is capable of interfacing between atmosphere pressure and vacuum where aerosol mass-spectrometry analysis may be performed [7]. In particular an aerosol focusing system design is needed that incorporates aerodynamic lens stack focusing technology with high flow atmospheric pressure sampling and delivers a tightly focused particle beam in vacuum within, for example, 300 μm for particles ranging from 1 μm to 10 μm. Furthermore, what is also needed is a design tool for dimensioning and validating the AFS (including various components of the AFS individually, such as the lens stack) so that various interface systems could be designed rapidly for different operating conditions without the need of lengthy computational fluid dynamic and costly bench top experimentation.